This invention relates to a light chopper assembly and in particular to a light chopper suitable for use in a video endoscope or borescope capable of producing either a black and white picture or a full color picture of a target.
Many endoscopes and borescopes are now video equipped to provide television pictures of a remote target. A light source is used to illuminate a target region within the viewing plane of the system's optics. White light from a source is transmitted through a probe into the target region and reflected light from the target is focussed by the optics upon an imager where the image information is converted to television compatible electrical signals. A more detailed description of this type of system is provided in U.S. Pat. Nos. 4,532,918 and 4,539,586.
In many of these endoscopeo or borescope systems a filter wheel is used to generate sequential color separated images of the target. The wheel is arranged to rotate through the beam emitted by the light source to project, in sequence, light of separate colors incident or the target. Reflected light from the target is received by the imager as color separated images. The field sequential color video signals are then brought together in a video processor to produce a full color picture of the target.
The color wheel is typically configured with separate filters spaced apart in a circular pattern. The regions between the filters do not transmit light, and hence these regions act as light shutters as used in a conventional camera. During the period that light from the source is being blocked, the image processor, having received a color separated image, is being read out by conventional video processing circuitry to produce a video picture. This type of system is further disclosed in U. S. Pat. Nos. 4,546,379 and 4,523,224.
To convert a color system of this type to a black and white system, unfiltered white light from a source may be transmitted directly to the target region. However, the need for the shutter function still remains. This function can be satisfied by simply removing the filters from the wheel to provide clear windows through which light can pass as the wheel rotates through the light beam. The regions between the windows chop the light and permit the images to be processed sequentially.
In many endoscopic or borescope applications, optimum color pictures can only be acquired when the target is within twelve inches of the imager. Targets located further away from the imager are, better viewed as black and white pictures. In practical applications, however, the target range may be anywhere from zero to three feet. Therefore, for optimum image viewing, both color and black and white systems are required. Apparatus for converting a video endoscope or borescope from a color system to a black and white system is described in co-pending U.S. patent application Ser. No. 221,776, filed July 20, 1988, now U.S. Pat. No. 4,862,253 in the name of English, et al. In the English device, a series of filter windows are circumferentially mounted about a wheel and a similar series of clear windows are positioned radially adjacent to the filter windows. A mechanism is provided which permits a light source's position relative to the wheel to be selectively changed. When in one position the source beam is sequentially passed through filter windows containing red, green and blue filters as the wheel rotates to produce a full color picture of a target. When in a second position the source beam passes through the clear windows to produce a black and white picture of the target.
The system for selectively converting a video equipped endoscope or borescope to produce either color or black and white pictures functions quite well in practice. It has, however, been found difficult to fabricate light chopping wheels suitable for use in this type of conversion system. The conversion wheels tend to be relatively heavy. A good deal of inertia is built up when a heavy wheel is turning at high speed which can cause the system to go out of synchronization. Resynchronization of this system may take up to fifteen seconds to accomplish, which in certain endoscopic applications, cannot be tolerated. The use of lightweight plastic wheels has not been totally satisfactory because the high operating temperatures of the lamps adversely effects many lightweight plastic materials. In addition, mounting color filters in a wheel has heretofore required a number of machining steps that were both time consuming and expensive.